How to Grow Petunias: A Practical Guide from Seed to Flowers

Petunias are hard to beat when you want plenty of summer colour. They are one of our favourite flowers with Petunia Pirouette being top of our list of favourites. They work brilliantly in pots, hanging baskets, window boxes and borders, and once they start flowering, they'll keep going for months with a little attention.

Growing petunias from seed takes a bit more patience than something like sunflowers or sweet peas. The seeds are incredibly small and the young seedlings don't exactly race away at first. Don't let that put you off, though. Once the plants get established and the weather warms up, they soon make up for their slow start.

The main things petunias need are warmth when they're young, plenty of light, a sunny position and regular feeding and watering once they're flowering. Keep removing faded flowers too, particularly on the larger-flowered varieties.

There are plenty of types to choose from, from compact plants covered in smaller flowers to trailing varieties for baskets and some wonderfully large and unusual blooms.

When to Sow

Petunias need quite a long growing season, so they're worth starting earlier than many of our other summer flowers.

I generally think of January to March as the main sowing period. January and February sowings are useful if you have somewhere warm and, importantly, plenty of light to grow the seedlings on. If you're relying on an ordinary windowsill, there's nothing wrong with waiting until March or April when the days are noticeably longer.

There's very little point sowing early if the seedlings are going to struggle for light afterwards.

Petunias need warmth to germinate, somewhere around normal warm indoor or propagator temperatures. A heated propagator makes life easier, but a warm, bright position indoors can work perfectly well.

Don't be discouraged if the seedlings initially seem tiny. They grow slowly at first.

The important thing is to get healthy, compact plants ready for late spring rather than enormous plants far too early.

How to Sow

Petunia seeds are extremely fine, so this is one of those jobs where I wouldn't attempt to sow them outside directly into the garden.

Fill a seed tray or small pot with fine seed compost, gently firm the surface and water it before sowing. Let the excess water drain away.

Sprinkle the seeds as evenly as you can over the surface. Don't worry if they're not perfectly spaced — with seed this small, perfection is asking rather a lot.

Petunia seeds need light to help them germinate, so don't bury them under a thick layer of compost. Press them gently onto the surface instead.

Cover the tray with a clear propagator lid or similar cover to help maintain humidity and put it somewhere warm and bright, but out of fierce direct sunshine.

Keep the compost moist rather than soaking wet. I prefer watering carefully from below where possible because a heavy stream from a watering can can move those tiny seeds and seedlings around.

Germination usually takes around one to two weeks in good conditions, although some batches will take a little longer.

Once seedlings appear, remove the cover gradually and give them as much good light as you can.

Growing Seedlings On

This is probably the stage that requires the most patience.

Petunia seedlings start off incredibly small. Once they're large enough to handle and have developed proper leaves, carefully move them into individual small pots or trays

Handle seedlings by a leaf rather than gripping the stem as they are easily damaged

Keep the young plants somewhere bright and frost-free. Good light is particularly important because warm conditions combined with poor light can give you weak, stretched plants.

Water carefully and don't leave the compost permanently saturated. Small petunias don't have a huge root system and sitting in cold, wet compost won't do them any favours.

As the plants develop, pinch out the growing tip if they're becoming tall rather than branching. This encourages side shoots and helps produce a fuller plant with more flowering stems.

Pot them on as necessary, but don't jump from a tiny seedling straight into an enormous pot. Moving up gradually makes watering much easier.

By late spring, you should have sturdy, branching plants ready to harden off.

Planting Out

Petunias are tender plants, so wait until the danger of frost has passed, which for many gardens means sometime from the middle or for us the end of May onwards. Local conditions matter, so keep an eye on the weather forecast or ask a local expert.

Before planting, harden the plants off over a week or two. Put them outside during the day in a sheltered position and bring them back under cover at night, gradually increasing the time they spend outdoors.

Choose a sunny position. Petunias will tolerate a little shade, but you'll generally get far more flowers in good sunshine.

They're particularly good in containers, hanging baskets and window boxes. Use decent multipurpose compost and make sure containers have drainage holes.

Spacing will depend on the variety, but around 20–30cm apart is a reasonable guide for many types. Trailing varieties in baskets can be arranged to spill naturally over the sides.

Water everything thoroughly after planting and give the roots time to establish before expecting the plants to fill their space.

Growing On

Once the warmer weather arrives, petunias really begin to get going.

Keep an eye on plants during the first few weeks after planting and make sure they don't dry out while establishing. Once they're growing strongly, you'll start to see why they're such popular summer bedding plants.

Some varieties naturally make compact, bushy plants, while trailing petunias will begin spilling over the sides of containers and baskets.

If plants become straggly, don't be frightened to trim them back. Petunias can respond remarkably well to a light haircut, producing fresh shoots and another flush of flowers.

Remove yellowing or damaged foliage as you find it and keep the centre of larger plants reasonably open.

Petunias can become sticky to touch, particularly around the stems and leaves. That's perfectly normal, so don't assume you've discovered something horribly wrong with them.

The other job is deadheading. Remove faded flowers and, importantly, the developing seed pod behind them. If you only pull away the colourful petals, the plant can still put energy into making seed.

Watering and Feeding

Petunias are hungry plants once they're in full growth, particularly when they're growing in containers.

Water regularly during warm weather, allowing excess water to drain away. Hanging baskets can dry astonishingly quickly on a sunny or windy day, so check them frequently rather than assuming yesterday's watering will be enough.

Try to water the compost rather than continually soaking the flowers. Large petunia blooms can look rather sorry after being repeatedly drenched.

Plants growing in the ground usually need less attention than those in pots, but they'll still appreciate a thorough watering during prolonged dry weather.

Once plants are established and flowering, start feeding regularly with a liquid feed suitable for flowering plants. A high-potash feed, such as tomato feed, works well when used according to the instructions.

If a previously healthy container starts producing fewer flowers and pale growth in midsummer, check both feeding and watering. By then, a large petunia can have filled much of its container with roots and will be relying on you for both.

Flowering and Deadheading

Petunias can flower from early summer right through into autumn, often continuing until the first proper frosts finish them.

Regular deadheading helps keep the display going.

Follow the faded flower back to its base and remove the developing seed pod as well as the old petals. It takes a little longer than simply pulling off the dead flower, but it stops the plant wasting energy producing seed.

Some modern varieties are better at shedding old flowers themselves.

If a plant becomes long, untidy and noticeably less floriferous halfway through summer, give some of the stems a trim. It may look slightly worse for a week or two, but fresh branching growth usually follows.

Keep feeding and watering after cutting back and you can often encourage an excellent second flush.

And don't be too quick to throw petunias away as summer starts drawing to a close. Given mild autumn weather, they can continue flowering surprisingly late.

Common Problems

Poor Germination

Petunia seed needs warmth, light and consistent moisture to germinate successfully. Don't bury the tiny seeds deeply, and don't allow the surface of the compost to dry completely while you're waiting.

Equally, avoid keeping the compost soaking wet. Warm, waterlogged compost is an excellent way to create problems rather than petunias.

Tiny Seedlings Not Growing

Petunias naturally start slowly, so give them time. Make sure they're receiving plenty of light and aren't sitting in cold, saturated compost.

Once they've developed a stronger root system and temperatures improve, growth becomes much quicker.

Slugs and Snails

Young petunias can be attractive to slugs and snails, particularly immediately after planting out. Check plants regularly while they're establishing. Larger plants can usually cope with a little nibbling far better than tiny seedlings.

Aphids

Aphids can gather around tender shoots and flower buds. Check plants occasionally, particularly underneath young growth. Small colonies can often be rubbed or washed away before they become a serious problem.

Yellow Leaves

Yellow foliage can result from several things, including overwatering, poor drainage, hungry plants or simply older leaves reaching the end of their life.

Look at the whole plant before reaching for the fertiliser. A couple of old yellow leaves at the base aren't the same as an entire plant gradually becoming pale.

Plants Becoming Leggy

Petunias can become long and straggly as summer progresses. Trim back some of the longer stems, continue watering and feeding, and the plant will usually produce fresh branching growth.

Flowers Going Brown

Heavy rain can make large petunia flowers look rather battered. Remove the damaged blooms and don't worry too much — another batch will usually be waiting behind them.